Independent control of branch FETs for RF performance improvement

ABSTRACT

A FET-based RF switch architecture and method that provides for independent control of FETs within component branches of a switching circuit. With independent control of branch FETs, every RF FET in an inactive branch that is in an “open” (capacitive) state can be shunted to RF ground and thus mitigate impedance mismatch effects. Providing a sufficiently low impedance to RF ground diminishes such negative effects and reduces the sensitivity of the switch circuit to non-matched impedances.

BACKGROUND

(1) Technical Field

This invention generally relates to electronic circuits, and morespecifically to radio frequency (RF) switch circuits utilizing fieldeffect transistors (FETs).

(2) Background

A recent trend in cellular radio architectures has been to incorporate amultiple pole FET-based RF switch architecture to accommodate multipleantennae, such that transmit and receive paths can connect to anyselected antenna. This added flexibility has led to more complicatedconfigurations of series and shunt RF FETs, requiring the need for noveltopological innovations to overcome performance degradations in multiplepole FET switches. For example, in an RF switch architecture, when RFFETs are turned off and have high impedance paths to RF ground, certainimpedance mismatch conditions can degrade signal insertion lossperformance.

As an example, FIG. 1 is an equivalent-element schematic diagram of atypical prior art FET-based RF switch circuit 100 having two branches.Elements M1-M8 are implemented as FET switches. With respect to appliedRF signals, each FET switch when “off” or “open” behaves as a capacitor,and when “on” or “closed” behaves as a conductive resistor. In theconfiguration shown. FET switches M1-M4 comprise a first branch 102 (fora port 104), and FET switches M5-M8 comprise a second branch 106 (forport 108). Either of the ports 104 or 108 may be coupled to antennas110, 112 to conduct transmitted or received RF signals to connectedcircuitry (not shown).

In the configuration shown in FIG. 1, branch 102 is active, and port 104is coupled to antenna 110 through FET switches M2 and M3, and isolatedby FET switch M1 from antenna 112. However, if M1 is “closed” and M2 is“open”, then port 104 is coupled to antenna 112 through M1 and M3, andisolated by FET switch M2 from antenna 110. In both cases, branch 106 isinactive and FET switches M5 and M6 are open, nominally isolating port108 from both antennas 110, 112. To couple port 108 to antenna 110. FETswitches M1-M4 are set to the states shown in FIG. 1 for FET switchesM5-M8, and FET switches M5-M8 are set to the states shown for FETswitches M1-M4. Note also that when a port is coupled to an antenna, theother antenna may be connected to a different port or be left “floating”(as in the example shown in FIG. 1). Not shown are the control linesthat synchronize state changes of the FET switches M1-M8, as describedabove. Of note is the fact that FET switches M7 and M8 are synchronouslycontrolled to be in complementary states, so that FET switch M7 isalways “open” when FET switch M8 is “closed, and vice versa.

A problem arises because the switch circuit 100 of FIG. 1 has acapacitive connection between the antennas 110, 112. In particular, forthe configuration shown in FIG. 1, there is a high impedance path fromnode X to ground through FET switches M7 and M8, since FET switches M5,M6, and M7 form a capacitor divider. In the illustrated state, thiscauses FET switches M5 and M6 to have a high impedance to RF ground whenin the “open” (capacitive) state and increases the performancesensitivity to load mismatch on antenna 112.

FIG. 2 shows the insertion loss 200 of a signal path to one antenna (indB) versus frequency for a 3:1 VSWR load on the unused antenna, sweptacross multiple phases, for a test implementation of the prior artswitch circuit 100 shown in FIG. 1. For the particular testimplementation of the switch circuit 100, the envelope of the insertionloss 200 showed a variability of up to 0.5 dB in the frequency range ofinterest (i.e., 0 GHz to 3 GHz in the illustrated example).

Accordingly, there is a need for a circuit and method for improving RFperformance of FET-based circuits used in RF switch architectures. Thepresent invention addresses this need.

SUMMARY OF THE INVENTION

The present invention includes a FET-based RF switch architecture andmethod that provides for independent control of FETs within componentbranches of a switching circuit. With independent control of FETs in abranch, every RF FET in an inactive branch that is in an “open”(capacitive) state can be shunted to RF ground and thus mitigate theimpedance mismatch effects of prior art architectures. Accordingly,providing a sufficiently low impedance to RF ground diminishes suchnegative effects and reduces the sensitivity of the switch circuit tonon-matched impedances.

In each branch of such a switch, a programmable shunt element isprovided that selectively couples the capacitive elements of the branchto ground when the branch is inactive (i.e., while another branch isactively coupling a signal to a destination element). In someembodiments, the inactive branches may be operated as a conventionalcircuit when impedance matching is not a problem for particularapplications.

The concept of independent control of programmable shunt FETs can beextended to the other FET switches of the branches. That is, rather thancontrolling the FET switches to change state in a rigidly synchronousfashion, all of the FET switches can be independently controlled,allowing unusual configurations of switch states that may have use inparticular applications. Accordingly, the invention is not limited toprogrammable control of only the shunt FET switches.

The inventive concepts also can be applied to a switching configurationthat comprises a single “branch”. For example, a node or port can beselectively coupled to two or more antennas, or completely disconnectedfrom any of the antennas, by opening or closing corresponding gatewayswitches programmatically. In addition, the node or port can be coupledto circuit ground as desired by closing a shunt switch programmatically,thus improving the isolation of the antennas from circuitry coupled tothe node or port through the gateway switches.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent-element schematic diagram of a typical prior artFET-based RF switch circuit having two branches.

FIG. 2 shows the insertion loss of a signal path to one antenna (in dB)versus frequency for a 3:1 VSWR load on the unused antenna, swept acrossmultiple phases, for a test implementation of the prior art switchcircuit shown in FIG. 1.

FIG. 3 is an equivalent-element schematic diagram of a FET-based RFswitch embodiment in accordance with the present invention.

FIG. 4 shows the insertion loss of a signal path to one antenna (in dB)versus frequency for a 3:1 VSWR load on the unused antenna, swept acrossmultiple phases, for a test implementation of the switch circuit shownin FIG. 3, with FET switches M5 and M6 shunted to ground through FETswitches M7 and M8.

FIG. 5 is a circuit diagram showing a second embodiment of theinvention.

FIG. 6 is a circuit diagram of the high-band SP3T switch shown in FIG.5, in a shunted state.

FIG. 7A is a circuit diagram of the high-band SP3T switch shown in FIG.5, in a non-shunted inactive state.

FIG. 7B is a circuit diagram of the high-band SP3T switch shown in FIG.5, in a non-shunted active state.

FIG. 8A is a schematic diagram of a switching circuit having a singlebranch configuration, depicting the various switch elements as schematicswitches.

FIG. 8b is a circuit diagram of the single branch switching circuit ofFIG. 8A, depicting the various switch elements as transistors.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes a FET-based RF switch architecture andmethod that provides for independent control of FETs within componentbranches of a switching circuit. With independent control of branchFETs, every RF FET in an inactive branch that is in an “open”(capacitive) state can be shunted to RF ground and thus mitigate theimpedance mismatch effects of prior art architectures. Accordingly,providing a sufficiently low impedance to RF ground diminishes suchnegative effects and reduces the sensitivity of the switch circuit tonon-matched impedances.

FIG. 3 is an equivalent-element schematic diagram of a FET-based RFswitch circuit 300 embodiment in accordance with the present invention.Elements M1-M8 are implemented as FET switches. With respect to appliedRF signals, each FET switch when “off” or “open” behaves as a capacitor,and when “on” or “closed” behaves as a conductive resistor. In theconfiguration shown, FET switches M1-M4 comprise a first branch 102 (fora port 104), and FET switches M5-M8 comprise a second branch 106 (forport 108). Either of the ports 104 or 108 may be coupled to antennas110, 112 to conduct transmitted or received RF signals to connectedcircuitry (not shown).

In the configuration shown in FIG. 3, branch 102 is active, and port 104is coupled to antenna 110 through FET switches M2 and M3, and isolatedby FET switch M1 from antenna 112. However, if M1 is “closed” and M2 is“open”, then port 104 is coupled to antenna 112 through M1 and M3, andisolated by FET switch M2 from antenna 110. Accordingly, PET switches M1and M2 behave as “gateway” switches to select a desired signal path tothe antennas 110, 112 for port 104.

Similar to branch 102. FET switches M5 and M6 in branch 106 behave as“gateway” switches to select a desired signal path to the antennas 110,112 for port 108. When branch 102 of the switch circuit 300 is active,branch 106 is normally inactive; accordingly, gateway FET switches M5and M6 are open, nominally isolating port 108 from both antennas 110,112. Further, FET switch M8 is closed, behaving as a shunt and couplingport 108 to circuit ground. However, in contrast to the prior art, FETswitch M7 of inactive branch 106 is not uniformly set to an “open”(capacitive) state when branch 102 of the switch circuit 300 is active.Instead. PET switch M7 is independently selectively settable (i.e.,programmable) to be in a “closed” (resistive) shunt state or in an“open” (capacitive) state. While control lines (not shown in general)are coupled to all of the FET switches M1-M8 to effectuate statechanges, the independent ability to control FET switch M7 (andcounterpart programmable shunt FET switches in other branches) isemphasized by showing the presence of an independent control element302.

When branch 102 of the switch circuit 300 is active and programmableshunt FET switch M7 in inactive branch 106 is in a “closed” shunt state,node X is shunted to ground through FET switches M7 and M8. This allowsFET switches M5 and M6 to be connected to RF ground when in the “open”(capacitive) state. For added flexibility, since FET switch M7 isindependently controllable, when FET switch M7 is in an “open” state,the switch circuit 300 behaves like existing designs.

When branch 106 of the switch circuit 300 is active to couple port 108to one of the antennas 110, 112, the FET switches of branch 102 areoperated in a reciprocal manner.

Note also that when a port is coupled to an antenna, the other antennamay be connected to a different port or be left “floating” (as in theexample shown in FIG. 3). As should be clear, additional switchingbranches may be added to provide additional switching capability, asindicated by the dashed lines in FIG. 3. Further, additional ports canbe added to each branch in known fashion.

By configuring the RF switch circuit 300 into independent, isolatedbranches and having finer control over state changes of the programmableshunt FETs of each branch through the independent control elements 302,the gateway FET switches M1/M2 and M5/M6 shown in FIG. 3 can beselectively shunted to RF ground. Doing so improves isolation of theantennas 102, 104 through the gateway FET switches of inactive branches.

FIG. 4 shows the insertion loss 400 of a signal path to one antenna (indB) versus frequency for a 3:1 VSWR load on the unused antenna, sweptacross multiple phases, for a test implementation of the switch circuit300 shown in FIG. 3, with gateway FET switches M5 and M6 shunted toground through shunt FET switches M7 and M8. For the particular testimplementation of the switch circuit 300, the envelope of the insertionloss 400 showed a variability of around 0.2 dB in the frequency range ofinterest (i.e., 0 GHz to 3 GHz in the illustrated example),significantly reducing the insertion loss sensitivity to variousconfounding conditions and improving overall insertion loss performancein comparison to the variability of up to 0.5 dB for the prior artdescribed above.

FIG. 5 is a circuit diagram showing a second embodiment of theinvention. In this embodiment, a switch circuit 500 includes a pair ofantennas ANT1, ANT2 that may be selectively coupled through multipleports to other circuitry (not shown) through gateway switches 502, 504,506, 508 to single-pole, multiple throw switches (SPxT) 510, 512. Inthis example, one SPxT switch 510 is for a low hand of frequencies andis shown as an SP6T switch; the other SPxT switch 512 is for a high bandof frequencies and is shown as an SP3T switch. In each branch of theswitch circuit 500, the combination of the coupled gateway switches andan associated SPxT switch provides for a double-pole, multiple throwswitch architecture. The architecture can be extended to additionalpoles for additional antennas, additional branches for additional ports,and each of the SPxT switches may have more or fewer ports than shown.

Sub-diagram 520 in FIG. 5 shows the passive circuit equivalents to theopen and closed states of the FET switches shown in the various figures.As in FIG. 3, the switching elements may be implemented as FET switches.

FIG. 6 is a circuit diagram of the high-band SP3T switch 512 shown inFIG. 5, in a shunted state. Only three ports are shown for simplicity,and fewer or more ports may be used for particular applications. In thisembodiment, any of the transmit/receive ports T/R1-T/R3 may be coupledto the HB Node (corresponding to the same high band, or HR, node in FIG.5) by closing an associated series switch 602, 602′ and opening anassociated shunt switch 604. Conversely, any of the transmit/receiveports T/R1-T/R3 may be uncoupled from the HB Node by opening itsassociated series switch 602, 602′ and closing its associated shuntswitch 604, 606 (which is the state shown for transmit/receive portsT/R1, T/R2, and T/R3 in FIG. 6).

When a branch of the switch circuit 500 is inactive, then the SPxTswitch elements in that branch should be decoupled by opening all of thesignal paths. However, as noted above, doing so with conventionalarchitectures will make the “open” branches of the switch circuit 500behave as capacitive dividers. Applying the teachings of the presentinvention to the circuit shown in FIG. 6, the HB Node is connected toground by closing FET switches 602′ and 606 in tandem. Doing so makesthe combination of the switch 512 and its associated gateway switches506, 508 behave like the M5-M8 branch shown in FIG. 3. Thus, in contrastto the prior art, FET switch 602′ is not opened (making it behave like acapacitor), but instead independently closed (making it behave like aresistor) to match the state of FET switch 606, thus making the pair ofFET switches 602′, 606 behave as a shunt from the HB node to ground.

The invention provides additional flexibility by allowing embodiments toalso behave in a conventional manner. For example, FIG. 7A is a circuitdiagram of the high-band SP3T switch 512 shown in FIG. 5, in anon-shunted inactive state. This circuit in general behaves like thecircuit of FIG. 6, except that FET switches 602′ and 606 are programmedto be oppositely switched, such that one is open while the other isclosed, as in conventional designs. In the illustrated state, FET switch606 is closed and FET switch 602′ is open, thereby decoupling theassociated T/R3 signal port from the HB Node. The opposite state isshown in FIG. 7B, which is a circuit diagram of the high-band SP3Tswitch shown in FIG. 5 in a non-shunted active state. In thisconfiguration. FET switch 606 is open, and FET switch 602′ is closed,thereby coupling the associated T/R3 signal port to the HB Node.

As should be clear, a single integrated circuit embodiment of thearchitecture shown in FIG. 5 may selectively program the switches 510,512 to be in a non-shunted configuration (behaving like conventionalswitches) or to be in a shunted configuration (behaving in accordancewith the present invention) simply by controlling the state of the shuntFET switch (switch 602′ in the embodiments illustrated in FIG. 6 andFIG. 7).

Extensions of the Inventive Concepts

The concept of independent control of the programmable shunt FETs (e.g.,M3 and M7 in FIG. 3) can be extended to the other FET switches of thebranches. That is, rather than controlling the FET switches to changestate in a rigidly synchronous fashion, all of the FET switches can beindependently controlled, allowing unusual configurations of switchstates that may have use in particular applications. Accordingly, theinvention is not limited to programmable control of only the shunt FETswitches corresponding to M3 and M7 in FIG. 2, but to all of the FETs ofa branch.

Further, the independent control elements 302 shown in FIG. 3 cancontrol the state of an associated programmable shunt FET switch to beother than in binary “open” or “closed” states. Thus, in a particularapplication, it may be useful to vary the impedance of the associatedprogrammable shunt FET switch to be something between the impedancepresented by fully “open” (capacitive) or fully “closed” (conductive)states.

The inventive concepts can be applied to a switching configuration thatcomprises a single “branch”. For example, FIG. 8A is a schematic diagramof a switching circuit 800 having a single branch configuration,depicting the various switch elements as schematic switches; FIG. 8b isa circuit diagram of the single branch switching circuit of FIG. 8A,depicting the various switch elements as transistors. A node or port 802can be selectively coupled to two or more external (with respect to theswitching circuit 800) circuit elements 804, or completely disconnectedfrom any of the circuit elements 804, by opening or closingcorresponding gateway switches 806, 808 programmatically. The circuitelements 804 are shown as antennas in the illustrated embodiment by wayof example only, and only two antennas are shown for simplicity. Inaddition, the node or port 802 can be coupled to circuit ground asdesired by closing a shunt switch 810 programmatically, thus improvingthe isolation of the circuit elements 804 from circuitry coupled to thenode or port 802 through the gateway switches 806, 808.

Implementation Details

As should be readily apparent to one of ordinary skill in the art, theinvention can be implemented to meet a wide variety of possiblespecifications. Thus, selection of suitable component values are amatter of design choice. The switching and passive elements may beimplemented in any suitable IC FET technology, including but not limitedto MOSFET and IGFET structures. Integrated circuit embodiments may befabricated using any suitable substrates and processes, including butnot limited to standard bulk silicon, silicon-on-insulator (SOI), andsilicon-on-sapphire (SOS) processes.

Other variations of the invention may include additional circuitelements. For example, low-pass, high-pass, and/or notch filters, orvarious transmission line, resistive, capacitive, or inductive circuitelements (passive or active) may be combined with the switcharchitecture of the present invention. As another example, any singleFET switch may be implemented instead as a stack of series connectedFETs to provide improved resistance to electro-static discharge (ESD)events.

Method Embodiments

Another aspect of the invention includes a method for operating a switchcircuit, including the steps of:

STEP 1: providing a FET-based switch circuit including at least oneswitching branch, each switching branch including at least twoprogrammable gateway switches configured to be connected to externalcircuit elements, a common node coupled to the at least two programmablegateway switches, and a shunt switch connected to the node; and

STEP 2: programmatically shunting the node of at least one switchingbranch to circuit ground in order to selectively isolate such node withrespect to any external circuit elements connected to the programmablegateway switches of such switching branch;

Yet another aspect of the invention includes a method for operating amultiple-branch switch circuit, including the steps of:

STEP 1: providing a FET-based switch circuit including at least twoswitching branches; and

STEP 2: providing in each switching branch of the switch circuit a shuntelement for selectively isolating such switching branch when at leastone other switching branch is actively conducting an applied signal.

Still another aspect of the invention includes a method for switching acircuit including at least two independent switching branches, includingthe steps of:

STEP 1: providing in each switching branch at least two series connectedgateway switches, the gateway switches defining selectable signal paths;

STEP 2: providing in each switching branch at least one signal switchingcircuit coupled to a common node for the gateway switches forselectively coupling at least one associated applied signal to aselected signal path through the node when such switching branch isactive, and for coupling the node to circuit ground when such switchingbranch is inactive.

A number of embodiments of the invention have been described. It is tobe understood that various modifications may be made without departingfrom the spirit and scope of the invention. For example, some of thesteps described above may be order independent, and thus can beperformed in an order different from that described. It is to beunderstood that the foregoing description is intended to illustrate andnot to limit the scope of the invention, which is defined by the scopeof the following claims, and that other embodiments are within the scopeof the claims.

What is claimed is:
 1. A FET-based switch circuit including at least oneswitching branch, each switching branch including at least twoprogrammable gateway switches configured to be connected to externalcircuit elements, a common node coupled to the at least two programmablegateway switches, and a shunt switch connected to the node, wherein theshunt switch is programmatically activated to shunt such node to circuitground in order to selectively isolate such node with respect to anyexternal circuit elements connected to such programmable gatewayswitches.
 2. A FET-based switch circuit including at least twoindependent switching branches, each switching branch including: (a) atleast two series connected gateway switches defining selectable signalpaths; and (b) at least one signal switching circuit coupled to a commonnode for the gateway switches for selectively coupling at least oneassociated applied signal to a selected signal path through the nodewhen the switching branch is active, and for coupling the node tocircuit ground when the switching branch is inactive.
 3. A FET-basedswitch circuit including at least two independent switching branches,each switching branch including: (a) at least two series connectedgateway switches defining selectable signal paths; and (b) at least onesignal switching circuit coupled to a common node for the gatewayswitches for selectively coupling an associated applied signal to aselected signal path through the node when the switching branch isactive; wherein each signal switching circuit of a switching branchincludes a shunt element coupled to the node and a path leading tocircuit ground for selectively isolating such switching branch tocircuit ground when such switching branch is inactive.
 4. The FET-basedswitch circuit of claim 3 wherein the shunt element is a FET-basedswitch.
 5. The FET-based switch circuit of claim 3 wherein the shuntelement of the signal switching circuit of at least one switching branchis selectively configured to not isolate such switching branch when suchswitching branch is inactive.
 6. A method for operating a switchcircuit, including the steps of: (a) providing a FET-based switchcircuit including at least one switching branch, each switching branchincluding at least two programmable gateway switches configured to beconnected to external circuit elements, a common node coupled to the atleast two programmable gateway switches, and a shunt switch connected tothe node; and (b) programmatically shunting the node of at least oneswitching branch to circuit ground in order to selectively isolate suchnode with respect to any external circuit elements connected to theprogrammable gateway switches of such switching branch.
 7. A method forswitching a circuit including at least two independent switchingbranches, including: (a) providing in each switching branch at least twoseries connected gateway switches, the gateway switches definingselectable signal paths; and (b) providing in each switching branch atleast one signal switching circuit coupled to a common node for thegateway switches for selectively coupling at least one associatedapplied signal to a selected signal path through the node when theswitching branch is active, and for coupling the node to circuit groundwhen the switching branch is inactive.
 8. A FET-based switch circuitincluding at least one switching branch, each switching branchincluding: (a) at least two programmable gateway switch means configuredfor connection to external circuit elements; (b) a common node coupledto the at least two programmable gateway switch means; and (c) a shuntmeans connected to the node for programmatically shunting such node tocircuit ground in order to selectively isolate such node with respect toany external circuit elements connected to such programmable gatewayswitch means.
 9. A FET-based RF switch circuit including: (a) at leasttwo gateway switching means each defining at least two selectable signalpaths separated by an associated node; and (b) signal switching meanscorresponding to each gateway switching means and coupled to theassociated node of such gateway switching means for selectively couplingat least one associated applied signal to a selected signal path throughthe corresponding node when the corresponding gateway switching means isactive, and for coupling the corresponding node to circuit ground whenthe corresponding gateway switching means is inactive.
 10. The FET-basedRF switch circuit of claim 9 wherein the signal switching means of atleast one switching branch is selectively set to not isolate suchswitching branch when such switching branch is inactive.